Man, I feel for you. I bought a 72 Duster 340 that is a complete dog, a good 318 will leave this POS in the dust. First issue it has is that it will not rev past 4,500 RPM's, after going through the whole ignition system and replacing almost every component, rebuilding a 72 thermoquad, putting a new engine wiring harness, recurving the distributor etc... it is still a dog, less of a dog but a dog nonetheless and it still will not rev over 4500 rpm's. I finally came to the conclusion that the issue is with the valve springs, I know that the engine was rebuilt around 20years ago and someone must have changed the springs and not put them at the correct installed height and the valves just float as it nears 4500 RPM. Anyways waiting for the heat to come down to pull the engine and deal with it... I have though of getting rid of this dog of a car myself.